The story of Salim the adopted son of Abu Hudhayfa, who was breastfed by Abu hudhayfa’s wife, while Salim was older, is an authentic Hadith, narrated by Muslim and others. 

Breastfeeding was under the guidance of the Prophet , and that was because the situation required that Salim should have access to the couple’s home after he had grown up, especially because they considered him their son, before Allah SWA made adoption impermissible.

The hadith did not mention that she make him suckle milk from her breast while he is adult, rather it is said that she extracted the milk and then gave it to him to drink. This is confirmed in a narration by Ibn Sa’d related from Al Waqidi from Muhammad bin Abdallah ibn Akhi Al zahri on the authority of his father who said: Sahla used to extract her milk into a bowl enough for one feeding, and Salim would drink it everyday until five days had passed, after that he could enter on her when her head was uncovered, which was allowed by the Prophet for Sahla. 

For this reason some of the scholars have said: It is allowed to do this only if necessarily needed, and it was not abrogated. This is an opinion which Sheikh Al Islam ibn Taymiyyah referred to when he said: it is a valid opinion.